1. Field of the Invention
The present invention relates to a semiconductor package including a metal alloy substrate, a semiconductor device requiring a high degree of airtightness which is disposed on the substrate, a plurality of leads bonded to the substrate by hermetic sealing, and a sealing cover bonded to the substrate through a seal ring. The invention also relates to a method for manufacturing such a semiconductor package.
Priority is claimed on Japanese Patent Application No. 2004-89141, filed Mar. 25, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
A metal, hermetically sealed package is used for enclosing electronic components such as semiconductor devices which require a high degree of airtightness. Familiar examples include box-type butterfly packages, can-shaped stem-type packages (stem-type semiconductor carriers), and small, MiniDil-type packages. Lead wires or lead terminals, referred to collectively below as “leads,” for carrying external electrical signals into the package (or carrier) are hermetically sealed at places where the leads enter the package (or carrier).
Such hermetic sealing is carried out to maintain an electrically insulated state between the metal package and the leads, and to airtightly seal the package interior. The metal substrate used in the metal package (carrier) is generally made of iron, stainless steel, iron-nickel alloy or the iron-nickel-cobalt alloy known by the trade name Kovar. These metals have sufficed for maintaining a high airtightness because of their good wettability with hermetic sealing glass and because they have a coefficient of thermal expansion similar to that of the glass.
Japanese Patent Publication No. 10-7441 (JP-A 10-7441) describes one technical approach for carrying out such hermetic sealing. In this prior art, a first metal layer made of nickel is formed on the surface of a metal terminal, then a second metal layer is formed on the nickel layer. During heat treatment, the nickel metal of which the first metal layer is made diffuses through the metal of the second metal layer, ultimately reaching the surface of these plating layers, where the nickel undergoes a redox reaction with glass, thereby forming a bonding layer. As a result, a strong hermetic seal can be achieved even on metal terminals having thereon a second metal layer to protect the surface.
However, with the increase over the past few years in the amount of heat generated by semiconductor devices, prior-art packaging materials are no longer capable of carrying out sufficient heat dissipation. Therefore, to prevent semiconductor device failure from heat generation by the device, tungsten-copper (WCu) alloys, tungsten-silver (Wag) alloys, molybdenum-copper (MoCu) alloys and molybdenum-silver (MoAg) alloys, all of which are endowed with a high heat conductivity and low thermal expansion characteristics, have come to be used as packaging materials in order to rapidly and efficiently dissipate the generated heat to the exterior. When a tungsten-copper alloy, tungsten-silver alloy, molybdenum-copper alloy or molybdenum-silver alloy is used as the substrate in a package (carrier), it has been necessary to braze a seal ring made of iron-nickel alloy or Kovar to the substrate in order to weld a cover (e.g., a lid or can) made of iron-nickel (FeNi) alloy or Kovar to the substrate.
When the seal ring made of iron-nickel alloy or Kovar is bonded to the substrate, the tungsten-copper alloy, tungsten-silver alloy, molybdenum-copper alloy or molybdenum-silver alloy is nickel-plated or otherwise pretreated to facilitate flow by the braze. In addition, heat treatment is carried out to increase adhesion between the substrate and the nickel plating. The leads are then hermetically sealed at lead entry points on the nickel-plated and heat-treated substrate, following which the seal ring is brazed to a predetermined position on the substrate.
However, because the metals making up the tungsten-copper alloy, tungsten-silver alloy, molybdenum-copper alloy or molybdenum-silver alloy are not mutually in solid solution, the surface condition of each of these alloys varies locally, which makes it difficult to induce uniform growth of the nickel plating. As a result, due to thermal excursions from heat treatment, the hermetic sealing step and the brazing after nickel plating, adhesion between the substrate and the nickel plating is degraded, making it difficult to maintain a high airtightness. Such packages (carriers) are thus unfit for housing semiconductor devices requiring a high degree of airtightness.